1. Field of the Invention
The present invention relates to a focus control method, more particularly relates to a focus control method for controlling the focus of an optical apparatus optically measuring or optically inspecting a physical quantity of a photo-mask, phase shift mask, etc.
2. Description of the Related Art
In the process of production of semiconductor devices or liquid crystal devices, mask patterns formed on a photo-mask are transferred onto a semiconductor wafer by an exposure apparatus. Various types of devices are produced through a development process, an etching process, and various other processes. Along with miniaturization of LSIs, the widths of the patterns of the photo-mask used in the exposure process have been miniaturized as well. Use is being made of phase shift masks as the photo-masks to deal with this miniaturization. Phase shift masks are designed so that a phase difference of π or an odd number times of that is formed between light passing through adjacent pattern elements. The resolution ends up falling from the designed value along with deviation. For this reason, in order to secure the quality of a phase shift mask, the phase shift amount of the phase shift mask is measured by using a phase shift measurement device.
As a device for measuring the phase shift amount of the phase shift mask of a half tone type, a phase shift measurement device using a two-light flux interferometer is known. In this measurement device, light transmitted through a phase shifter interferes with light transmitted through a portion where the half tone film is not formed, and the interference light is received by a photo-detector. Then, the phase shift amount is measured based on an output signal from the photo-detector (see for example Japanese Patent Publication (A) No. 11-327119).
The device for measuring the phase shift amount by using a shearing interferometer can correctly measure the phase shift amount of the phase shifter. On the other hand, along with miniaturization of the LSIs, the diameter of the openings formed in the half tone film has been miniaturized as well. Openings of about 1.0 μm are formed in the half tone film. When the phase shift amount of a phase shift mask having such minute openings formed therein is optically measured, the diffraction phenomenon strongly acts and the phase shift amount to be measured greatly fluctuates even when the focus state of the measurement device slightly displaces from the normal focus position. When the present inventors actually measured the relationship between the focus displacement amount of an optical system and the measured phase shift amount for the phase shift mask having openings with a variety of diameters formed therein, it has found that when the diameters of the openings became 3 μm or less, the measured phase shift amount greatly changed by exactly the very small displacement of the focus of the optical system.
On the other hand, in the phase shift measurement device currently being practically used, the focus of the shearing interferometer is manually adjusted out by an operator while viewing a pattern image displayed on a monitor. For this reason, the actual focus position of the objective lens sometimes displaces from the true focus position, so the measured phase shift amount deviates from the true phase shift amount. This occurs in the same way not only in a case of measuring the phase shift amount of the phase shift mask, but also in a case of optically inspecting for defects and measuring the physical quantities of semiconductor wafers and various types of photo-masks.